1. Field of the Invention
The present invention relates generally to digital data recording on a disk (magnetic, magneto-optical, or optical) having information-storing tracks grouped into annular zones in which the ratio of the radius of a given zone to the corresponding radius of the adjacent inner zone is a constant which is the same for each adjacent pair of zones. Thus, when the ratio of the angular speed of one zone to the speed of the adjacent outer zone is the same constant, the linear velocity of each track falls within a range which is constant from one zone to the next. More particularly, the invention relates to the distribution of prerecorded indicia on the disk for phase locking a write clock so as to write data subsequently between the indicia.
2. Description Relative to the Prior Art
With reference to the recording art, tracking is the process of keeping a transducer head on the path of a record track on a storage device. The purpose of tracking control is to adjust the position of the head relative to the record track or vice versa, so that the head is aligned with the track for maximizing the signal-to-noise ratio of a data channel.
A technique known in the prior art for tracking includes writing tracking information, hereinafter referred to as a tracking pad, at positions spaced along a record track prior to writing user data on the disk. When the tracking pads are uniformly spaced on a disk, for example, each pad functions conveniently to provide a clock synchronization pulse which serves for recording user data between adjacent tracking pads even in the presence of random fluctuations in the velocity of the disk.
Each tracking pad is distinguishable from recorded user data and thereby permits tracking control apparatus to also maintain a head on a track centerline during a playback mode. For that purpose, each tracking pad is represented by a recording pattern which is readily distinguishable from any normal delay modulation mark (DMM) or modified frequency modulation (MFM) pattern corresponding to encoded data recorded between the pads.
With a disk, however, which has record tracks grouped into a plurality of annular zones arranged according to the "ratio" method, as disclosed in the aforementioned U.S. patent application Ser. No. 900,197, the frequency of occurrence of the tracking pads during a data-writing operation is generally not an integer number per each revolution. Thus, tracking pads on any one track within any given zone do not necessarily align radially with tracking pads on other tracks in the same zone. Accordingly, when a write transducer head moves from one track to any other track in the same zone, there is a discontinuity in the phase relationship between the first tracking pad accessed when entering the "new" track and the last tracking pad encountered when leaving the "old" track. With a write clock (or read clock) phase locked to the tracking pads, a track-jump operation causes an incorrect frequency of the clock as the phase locked loop (PLL) attempts to make up for this disturbance. Until the correct frequency is reacquired, data transfer (either recording or playback) must be interrupted which, of course, is disadvantageous as it slows the average speed over which data may be transferred to and from the disk. If, on the other hand, the frequency were not corrected, user data would be written over the tracking pads, thereby potentially disrupting a subsequent tracking operation and possibly rendering user data unreadable.